Noise canceller

ABSTRACT

If a mode calculating section ( 52 ) judges that an inputted modulated signal is nearly in a no-sound state, the interpolation width determined by an interpolation width calculating section ( 53 ) is increased, whereas if the mode calculating section ( 52 ) judges that the inputted modulated signal contains a lot of high-frequency components, the interpolation width determined by the interpolation width calculating section ( 53 ) is decreased.

TECHNICAL FIELD

The present invention relates to a noise canceler for canceling noisecontained in an inputted signal, and more particularly to a noisecanceler as incorporated in an FM receiver apparatus to cancel noisecontained in an FM reception signal.

BACKGROUND ART

In a car-mounted FM receiver apparatus, the received FM reception signalhas pulse noise such as ignition noise superimposed thereon, andtherefore, for the purpose of canceling such pulse noise contained inthe FM reception signal, there is provided a noise canceler.

With a conventional noise canceler, when a composite signal having pulsenoise superimposed thereon as shown in FIG. 6A is received, the pulsenoise is detected by passing the composite signal through a HPF(high-pass filter). When the HPF detects the pulse noise, a pulse noisedetection signal as shown in FIG. 6B is produced. When the pulse noisedetection signal is fed to an integrator, the integrator yields anoutput as shown in FIG. 6C.

Specifically, when the integrator is fed with the pulse noise detectionsignal, a capacitor included in the integrator is charged, making theoutput of the integrator higher than a threshold value. When the outputof the integrator becomes higher than a predetermined threshold value inthis way, the integrator is so controlled that the capacitor isdischarged, causing the output of the integrator to decrease gradually.By comparing the output of this integrator and the predeterminedthreshold value, a gate control signal is produced. With this gatecontrol signal, the operation of a gate circuit for canceling the pulsenoise is controlled.

Thus, when the output of the integrator is as shown in FIG. 6C, a pulsenoise detection signal is produced, and while the output of theintegrator remains higher than the threshold value, the gate controlsignal remains high. The gate circuit performs signal processing suchthat the signal level of the composite signal is held at its signallevel immediately before the occurrence of the pulse noise. As a result,the composite signals is output after having the pulse noise cancelledtherefrom as shown in FIG. 6D.

However, in a noise canceler that cancels pulse noise by operating asshown in FIGS. 6A to 6D, during the occurrence of pulse noise, the gatecircuit maintains the signal level immediately before the occurrence ofpulse noise irrespective of the state of the reception signal. Thiscauses distortion in the reception signal, resulting in unsatisfactoryquality of the sound reproduced therefrom.

Incidentally, Japanese Patent Application Laid-Open No. H8-56168proposes an FM receiver apparatus that switches filters according to thestate of reception, wherein the gate period during which the signallevel of the reception signal is maintained to cancel pulse noise isvaried so as to achieve appropriate cancellation of the pulse noise.This method, however, is no different from the operations illustrated inFIGS. 6A to 6D in that the pulse noise is cancelled by maintaining thesignal level immediately before the occurrence thereof, causingdistortion in the reception signal.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a noise canceler thatperforms interpolation according to the state of the reception signalafter canceling pulse noise therefrom.

To achieve the above object, according to the present invention, a noisecanceler that includes a pulse position determining section fordetecting pulse noise superimposed on an audio signal and that cancelsfrom the input signal the pulse noise detected by the pulse positiondetermining section is provided with: a state calculating section thatevaluates the state of the audio signal; an interpolation widthcalculating section that, according to the proportion of high-frequencycomponents contained in the audio signal as evaluated by the statecalculating section, sets an interpolation width at the data position atwhich to cancel the pulse noise and perform interpolation; and a pulsenoise reducing section that processes the data present within theinterpolation width set by the interpolation width calculating section,with the center of the interpolation width located at the data positionat which the pulse position determining section has detected the pulsenoise from the audio signal, so as to cancel the pulse noise and performinterpolation and that then outputs the audio signal thus processed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the internal configuration of an FMreceiver apparatus incorporating a noise canceler according to theinvention.

FIG. 2 is a block diagram showing the internal configuration of thenoise canceler according to the invention.

FIG. 3 is a diagram showing the operation performed to achievecorrection.

FIGS. 4A to 4C are diagrams showing how a modulation signal in ano-sound state is corrected.

FIGS. 5A to 5C are diagrams showing how a modulation signal of asinusoidal wave having a frequency of 3 kHz is corrected.

FIGS. 6A to 6D are diagrams showing various signals illustrating theoperation of a conventional noise canceler.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. FIG. 1 is a block diagram showing theinternal configuration of an FM receiver apparatus incorporating a noisecanceler according to the invention. FIG. 2 is a block diagram showingthe internal configuration of the noise canceler according to theinvention.

The FM receiver apparatus shown in FIG. 1 includes: an antenna 1 forreceiving broadcast signals; a front-end section (FE) 2 for selecting,from among the broadcast signals received by the antenna 1, the FMreception signal of a desired channel and subjecting it to RFamplification; an intermediate frequency amplifier section (IF) 3 forconverting the FM reception signal selected by the FE 2 to anintermediate frequency of 10.7 MHz and amplifying it; a detector section4 for extracting a modulation signal by detecting the FM receptionsignal that have been subjected to frequency conversion by the IF 3; anoise canceler (NC) 5 for canceling noise superimposed on the modulationsignal obtained through detection by the detector section 4; amultiplexer (MPX) 6 for separating the modulation signal having noisecancelled therefrom by the NC 5 into audio signals to be fed to a leftand a right speaker 7 and 8; and a left and a right speaker 7 and 8 fromwhich to reproduce sound.

When the FM reception signal of a desired channel frequency is selectedby the FE 2 from among the broadcast signals received by the antenna 1,then, in the IF 3, the selected FM reception signal is mixed with alocal oscillation signal and is thereby converted to an intermediatefrequency. Then, in the detection section 4, the FM reception signalthus converted into an intermediate frequency is detected by a detectionmethod such as one based on a phase-locked loop to obtain a modulationsignal. Moreover, in the detection section 4, the modulation signal isconverted into a digital signal. This modulation signal is then fed tothe NC 5, where the noise superimposed on the modulation signal isdetected and cancelled. The modulation signal thus having noisecancelled therefrom is then fed to the MPX 6, which processes the mainand sub channel signals contained in the modulation signal to separateit into audio signals to be fed to the left and right speakers 7 and 8,and then feeds those audio signals to the left and right speakers 7 and8.

Now, the NC 5 incorporated in this FM receiver apparatus will bedescribed. The NC 5 shown in FIG. 2 includes a pulse positiondetermining section 51 for detecting pulse noise superimposed on themodulation signal obtained from the detection section 4; a modecalculating section 52 for evaluating the state of the modulationsignal; an interpolation width calculating section 53 for setting,according to the state of the modulation signal as evaluated by the modecalculating section 52, an interpolation width within which to performinterpolation around the position at which the pulse noise has beendetected; and a pulse noise reducing section 54 for canceling the pulsenoise detected by the pulse position determining section 51 andperforming interpolation after cancellation of the pulse noise.

When the modulation signal in the form of discrete digital signals isfed to the NC 5, the pulse position determining section 51 detects theposition at which the pulse noise is superimposed on the modulationsignal. Here, for example, the modulation signal is first filtered witha high-pass filter, and is then formed into an absolute value with anabsolute value circuit. The modulation signal thus formed into anabsolute value is then passed through a limiter circuit so that aportion thereof with an extremely large amplitude is removed therefrom,and is then fed to a time-average circuit to calculate the time average.Then, the signal level of the modulation signal formed into an absolutevalue is compared with the time average, and, if the signal level issufficiently high relative to the time average, pulse noise isrecognized to be occurring, and its position is detected.

Incidentally, the inventor of the present invention proposed the detailof the pulse position determining section 51, for example, in JapanesePatent Application Laid-Open No. 2001-102944, titled “Noise DetectionApparatus in a Radio Receiver.” In the embodiment under discussion, thepulse position determining section is assumed to be based on the noisedetection apparatus proposed in the Japanese Patent ApplicationLaid-Open No. 2001-102944. Needless to say, however, the pulse positiondetermining section may be configured in any other manner.

In the mode calculating section 52, first, the modulation signal fedthereto is squared to form a squared value, and the amplitude of themodulation signal during a predetermined period is measured. Themeasured amplitude of the modulation signal is compared with apredetermined threshold value, and, if the amplitude remains lower thanthe threshold value during the predetermined period, the modulationsignal is recognized to be nearly in a no-sound state. If the modulationsignal is recognized not to be in a no-sound state, then the ratio ofthe high-frequency components filtered from the modulation signal by aHPF during the predetermined period to all the components of themodulation signal is calculated, and, if the calculated ratio is greaterthan a predetermined value, the modulation signal is judged to contain ahigh proportion of high-frequency components.

In this way, in the mode calculating section 52, first, whether or notthe input modulation signal is in a first mode, i.e., nearly in ano-sound state, is checked. Next, if the input modulation signal isfound not to be in the first mode, whether it is in a second mode, inwhich it contains a low proportion of high-frequency components, or in athird mode, in which it contains a high proportion of high-frequencycomponents, is checked. Thus, the mode calculating section 52distinguishes between three modes, namely the first to third modes.

Having distinguished between the three modes, the mode calculatingsection 52 notifies the interpolation width calculating section 53 ofthe recognized mode. The interpolation width calculating section 53 setsthe interpolation width over which to cancel noise; that is, it sets theinterval of time over which, when pulse noise is detected, interpolationis performed for waveform shaping after cancellation of the pulse noise.Here, if the mode calculating section 52 has recognized the first mode,the interpolation width is set to be longest, and, if the modecalculating section 52 has recognized the third mode, the interpolationwidth is set to be shortest.

Here, since the modulation signal is fed in in the form of sampleddiscrete signals, the interpolation width is set in terms of the numberof data over which to perform interpolation. Specifically, in the firstmode, the interpolation width encompasses ten data including the one atwhich the pulse noise was detected; in the second mode, theinterpolation width encompasses seven data including the one at whichthe pulse noise was detected; and, in the third mode, the interpolationwidth encompasses five data including the one at which the pulse noisewas detected.

Moreover, in the pulse noise reducing section 54, to remove thediscontinuity between the portion where interpolation was performed andthus pulse noise was cancelled and the remaining portion where nointerpolation was performed, the modulation signal is then subjected toprocessing with a LPF (low-pass filter). The cut-off frequency of theprocessing with the LPF here is set according to the mode. Specifically,the cut-off frequency is set to be lowest in the first mode and highestin the third mode.

Then, the data position at which the pulse noise is superimposed asdetected by the pulse position determining section 51 and theinterpolation width set by the interpolation width calculating section53 at the data position at which the detected pulse noise issuperimposed are fed to the pulse noise reducing section 54. Then,linear interpolation is performed by using the data preceding andsucceeding the interpolation width, with the center of the interpolationwidth located at the data position at which the pulse noise issuperimposed, so as to determine the data at each data position withinthe interpolation width.

For example, suppose that, as shown in FIG. 3, pulse noise is detectedat a data position Y3, and that the third mode is recognized, with theresult that the number of data within the interpolation width is set tobe five. Moreover, let the signal levels at the individual datapositions Y1 to Y5 within the interpolation width be y1 to y5, let thesignal level at the data position Xa immediately before theinterpolation width be xa, and let the signal level at the data positionXb immediately after the interpolation width be xb. Then, the signallevels y1 to y5 at the individual data positions Y1 to Y5 within theinterpolation width are set as follows:y1=(xb−xa)/6+xay2=2×(xb−xa)/6+xay3=3×(xb−xa)/6+xay4=4×(xb−xa)/6+xay5=5×(xb−xa)/6+xa

FIGS. 4A to 4C and FIGS. 5A to 5C show how this pulse cancellation isperformed in a no-sound state and with a sinusoidal wave having afrequency of 3 kHz, respectively. FIGS. 4A and 5A show the modulatedsignal having pulse noise superimposed thereon, FIGS. 4B and 5B show themodulated signal after being subjected to interpolation with the numberof data within the interpolation width set to be five, and FIGS. 4C and5C show the modulated signal after being subjected to interpolation withthe number of data within the interpolation width set to be ten. FIGS.4A to 4C and FIGS. 5A to 5C are diagrams showing how interpolation isperformed differently with different interpolation widths.

When pulse noise is superimposed on the modulation signal in a no-soundstate (in the first mode) as shown in FIG. 4A, if the number of datawithin the interpolation width is set to be five, the pulse noise is notcompletely cancelled from the modulation signal after interpolation asshown in FIG. 4B, leaving behind an uncorrected part of the pulse noise.In this case, by increasing the number of data within the interpolationwidth to ten, it is possible to completely cancel the pulse noise and inaddition restore the portion where the pulse noise was cancelled andinterpolation was performed to the no-sound state as shown in FIG. 4C.

On the other hand, when pulse noise is superimposed on the modulationsignal when it is a sinusoidal wave having a frequency of 3 kHz (in thethird mode) as shown in FIG. 5A, if the number of data within theinterpolation width is set to be ten, the modulation signal afterinterpolation and pulse noise cancellation is output with a distortedwaveform as shown in FIG. 5C. In this case, by reducing the number ofdata within the interpolation width to five, it is possible tocompletely cancel the pulse noise and in addition restore the portionwhere the pulse noise was cancelled and interpolation was performed toclose to a sinusoidal wave having a frequency of 3 kHz as shown in FIG.4C.

Thus, the higher the proportion of high-frequency components that themodulation signal contains is, the shorter the interpolation width needsto be set to be to achieve appropriate interpolation. In this way, inthe pulse noise reducing section 54, pulse noise is cancelled andinterpolation is performed; then, through processing using an LPF, thediscontinuity between the interpolated and non-interpolated portions isremoved. As a result, the pulse noise reducing section 54 outputs themodulation signal with reduced pulse noise and with alleviateddistortion resulting from correction.

In this embodiment, the mode calculating section distinguishes betweenthree modes, namely the first to third modes in which the modulationsignal is in a no-sound state, contains a low proportion ofhigh-frequency components, and contains a high proportion ofhigh-frequency components, respectively. It is, however, also possibleto use a plurality of types of filter to more finely distinguish betweendifferent states of the modulation signal. In that case, by setting theoptimum interpolation width for each state, it is possible to alleviatethe distortion that occurs in the modulation signal as a result ofinterpolation. Interpolation may be achieved by any other method thanlinear interpolation, which is simple. Instead of converting themodulation signal into a digital signal in the detector section, it maybe converted into a digital signal after being converted to anintermediate frequency so as to be subjected to digital signalprocessing in the circuit blocks succeeding the IF stage.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to adjust, accordingto the state of an input signal, the interpolation width over which toperform interpolation. This makes it possible to perform optimuminterpolation in different states. This helps alleviate the distortionthat occurs in the waveform of the input signal as a result ofinterpolation and thereby obtain a natural waveform. In a no-sound stateor a state close thereto, by increasing the interpolation width, it ispossible to prevent an uncorrected part of the superimposed noise frombeing left behind. On the other hand, when the proportion ofhigh-frequency components is high, by reducing the interpolation width,it is possible to alleviate the distortion in the waveform afterinterpolation. Moreover, by subjecting the input signal afterinterpolation to processing with an LPF, it is possible to alleviate thediscontinuity between the interpolated and non-interpolated portions.

1. A noise canceller including a pulse position determining section for detecting pulse noise superimposed on an audio signal, the noise canceller canceling from the input signal the pulse noise detected by the pulse position determining section, comprising: a state calculating section that evaluates state of the audio signal; an interpolation width calculating section that, according to a proportion of high-frequency components contained in the audio signal as evaluated by the state calculating section, sets an interpolation width at a data position at which to cancel the pulse noise and perform interpolation; and a pulse noise reducing section that processes data present within the interpolation width set by the interpolation width calculating section, with a center of the interpolation width located at a data position at which the pulse position determining section has detected the pulse noise from the audio signal, so as to cancel the pulse noise and perform interpolation and that then outputs the audio signal thus processed.
 2. A noise canceller as claimed in claim 1, wherein, in the interpolation width calculating section, if the audio signal is judged to be nearly in a no-sound state by the state calculating section, the interpolation width is set to be longest, and, the higher a proportion of high-frequency components that the audio signal is judged to contain by the state calculating section is, the shorter the interpolation width is set to be.
 3. A noise canceller as claimed in claim 2, wherein, in the state calculating section, first whether or not the audio signal is in a no-sound state is checked, and then whether or not the audio signal contains a high proportion of high-frequency components is checked.
 4. A noise canceller as claimed in claim 3, wherein, in the pulse noise reducing section, the audio signal having been subjected to interpolation is further filtered with a low-pass filter.
 5. A noise canceller as claimed in claim 4, wherein, in the interpolation width calculating section, the higher the proportion of high-frequency components that the audio signal is judged to contain by the state calculating section is, the higher a cut-off frequency of the low-pass filter is set to be before the audio signal is fed to the pulse noise reducing section.
 6. A noise canceller as claimed in claim 2, wherein, in the pulse noise reducing section, the audio signal having been subjected to interpolation is further filtered with a low-pass filter.
 7. A noise canceller as claimed in claim 6, wherein, in the interpolation width calculating section, the higher the proportion of high-frequency components that the audio signal is judged to contain by the state calculating section is, the higher a cut-off frequency of the low-pass filter is set to be before the audio signal is fed to the pulse noise reducing section.
 8. A noise canceller as claimed in claim 1, wherein, in the state calculating section, first whether or not the audio signal is in a no-sound state is checked, and then whether or not the audio signal contains a high proportion of high-frequency components is checked.
 9. A noise canceller as claimed in claim 8, wherein, in the pulse noise reducing section, the audio signal having been subjected to interpolation is further filtered with a low-pass filter.
 10. A noise canceller as claimed in claim 9, wherein, in the interpolation width calculating section, the higher a proportion of high-frequency components that the audio signal is judged to contain by the state calculating section is, the higher a cut-off frequency of the low-pass filter is set to be before the audio signal is fed to the pulse noise reducing section.
 11. A noise canceller as claimed in claim 1, wherein, in the pulse noise reducing section, the audio signal having been subjected to interpolation is further filtered with a low-pass filter.
 12. A noise canceller as claimed in claim 11, wherein, in the interpolation width calculating section, the higher a proportion of high-frequency components that the audio signal is judged to contain by the state calculating section is, the higher a cut-off frequency of the low-pass filter is set to be before the audio signal is fed to the pulse noise reducing section. 